Is that hot/cold, park/neutral, running/or not,and on level ground. Normally the trans has a 1 to 2 degree tail-down angle.
Are the stick and tube a matched set and is the stopper on the end correctly anchored?
The trick is to not have the sunshell, or any other spinning parts flinging oil.
As the trans cools off, the oil and air inside, contract, and the unit pulls in air as it stabilizes. Then as the unit heats next time out, the oil and air expand. The air vacates thru the vent, sometimes, if the oil is being aerated,that vacating air will take oil with it. The vent is baffled inside the trans ( or was when it left the factory), or it had a labyrinth separator, but it can only do so much.
I'm not saying your leak is the vent, only the vent, and nothing but the vent, lol. What I am saying, is; just cuz the stick says whatever, doesn't necessarily make it so.
Pipe plugs have to be tight to seal. But again, because they are tapered and a high-rate of leverage screw, which is a ramp, so; it can be easy to overtighten them. Your teflon-tape fix should be fine.
